Using Fiber Optics in a Broadband, Sensitive, Isotropic Antenna --15 kHz to 150 MHz

A broadband, active, isotropic receiving antenna was developed at NBS for the frequency range 15 kHz to 150 MHz. It was designed for use with a conventional receiver to measure weak, near-zone electric fields of unknown polarization, such as leakage emanations from electronic equipment placed within a shielded enclosure. The antenna system consists of three mutually-orthogonal active dipoles, each 31 cm long by 3 cm diameter. The entire frequency range of each of the three field components is amplified and used to modulate a high-speed light emitting diode (LED) located inside the dipole. The modulated infrared (IR) signals are guided through glass fibers 10 meters long which connect the "isolated" dipoles to avalanche photodiodes at the far end of the fiber guides. These photodetectors recover the RF modulation from the IR carrier for input to the receiver. The fiber-optic antenna system described in this paper has high sensitivity (down to 10 μV/m) and fast response time (RF modulations up to 150 MHz). The read-out indication at each receiver frequency is proportional to the Hermitian magnitude of E, which is the root-sum-square value of three orthogonal E field components at the measurement point. The linear dynamic range of the antenna system depends on the receiver bandwidth and signal frequency. It is 70 to 75 dB at frequencies between 0.02 and 2 MHz (for 0.5 kHz receiver bandwidth), 60 to.70 dB between 2 and 25 MHz (for 5 kHz receiver bandwidth) and 40 to 55 dB between 25 and 200 MHz (for 50 kHz bandwidth).